The fast-follow to the ifRev CAS fix: the sweep for the same check-then-act
class found two more instances, both now closed with the same discipline —
the decision runs at the serialization point that guards the apply.
add({ifAbsent}) / add({upsert}): the absence check ran before the commit
mutex, so N concurrent same-id creates could all pass it and all write — the
second silently overwriting the first, violating ifAbsent's "return the
existing id WITHOUT writing" contract and upsert's merge-never-clobber
contract. The insert leg now carries a must-be-absent commit precondition
(the same conditional-commit primitive ifRev uses), verified under the commit
mutex against the authoritative before-image. A losing caller takes its
documented resolution instead of overwriting: ifAbsent returns the existing
id with zero writes; upsert merges into the now-existing entity via update()
(shared param mapping in upsertMergeParams so the planning-time branch and
the conflict retry can never drift), with a bounded retry if a concurrent
delete lands between the conflict and the merge. The planning-time checks
remain as fast-fails. transact()'s batch-level ifAbsent/upsert keep
planning-time semantics (documented, converges to a valid entity).
BlobStorage: write()'s dedup decision (exists → increment refCount, absent →
create at 1) and delete()'s decrement-then-remove were unserialized
read-modify-writes over blob-meta:<hash>. Concurrent writes of identical
content could lose references, so a later delete removed bytes another file
still referenced (data loss), or leaked unreferenced blobs. All
reference-count-bearing mutations now serialize through a per-hash
InMemoryMutex — distinct content never contends; incrementRefCount/
decrementRefCount are documented lock-assumed internals.
Both fixes are complete by construction in-process: storage enforces
single-writer-per-directory, so the process is the whole concurrency domain.
Tests (tests/integration/ifabsent-upsert-blob-concurrency.test.ts): 8-way
ifAbsent storm advances the store by exactly one generation with _rev 1;
8-way upsert storm on an absent id yields one create + seven merges
(_rev === 8); sequential ifAbsent/upsert semantics pinned unchanged; N
identical concurrent blob writes → refCount === N; the blob survives until
the true last reference drops; interleaved write/delete storm never loses a
landed reference.
David Snelling
867939ed50